Leucine supplementation enhances integrative myofibrillar protein synthesis in free-living older men consuming lower- and higher-protein diets: a parallel-group crossover study

Background: Leucine co-ingestion with lower-protein (LP)-containing meals may overcome the blunted muscle protein synthetic response to food intake in the elderly but may be effective only in individuals who consume LP diets. Objective: We examined the impact of leucine co-ingestion with mixed macronutrient meals on integrated 3-d rates of myofibrillar protein synthesis (MyoPS) in free-living older men who consumed higher protein (HP) (1.2 g center dot kg(-1) center dot d(-1)) or LP (0.8 g center dot kg(-1) center dot d(-1)) in rested and resistance exercise (REX) conditions. Design: In a crossover design, 20 healthy older men [aged 65-85 y] were randomly assigned to receive LP or HP diets while ingesting a placebo (days 0-2) and Leu supplement (5 g leucine/meal; days 3-5) with their 3 main daily meals. A bout of unilateral REX was performed during the placebo and Leu treatments. Ingested (H2O)-H-2 and skeletal muscle biopsies were used to measure the 3-d integrated rate of MyoPS during the placebo and Leu treatments in the rested and REX legs. Results: Leucinemia was higher with Leu treatment than with placebo treatment (P < 0.001). MyoPS was similar in LP and HP during both treatments (P = 0.39) but was higher with Leu treatment than with placebo treatment in the rested (pooled mean +/- SD: Leu, 1.57% +/- 0.11%/d; placebo, 1.48% +/- 0.08%/d; main effect of treatment: P < 0.001) and REX (pooled mean: Leu, 1.87% +/- 0.09%/d; placebo, 1.71 +/- 0.10%/d; main effect of treatment: P < 0.001) legs. Conclusions: Leu co-ingestion with daily meals enhances integrated MyoPS in free-living older men in rested and REX conditions and is equally effective in older men who consume daily protein intakes greater than or equal to the RDA.